1. Technical Field
The present application relates to an antenna device which forms a beam having a radiation direction freely set by using a lens or a passive element acting as a lens, radiates the beam in the radiation direction and receives the beam reflected by an object to detect the bearing angle to the object.
2. Description of Related Art
An antenna device has been used to radiate a beam of electromagnetic waves while scanning the beam within a predetermined range of scanning angle. Further, this device receives the beam reflected by an object to detect the bearing angle to the object.
For example, a well-known Rotman lens with a Rotman lens pattern acting as wave-guiding channels is used for the antenna device. In this lens, electromagnetic waves induced from a transmission signal are distributed to form a beam directed in a radiation direction, and electromagnetic waves of an incoming beam are combined with one another to produce a reception signal indicating the incoming direction of the beam.
This Rotman lens has a channel pattern, a plurality of antenna ports disposed on one side of the lens, and a plurality of beam ports disposed on another side of the lens. In response to a transmission signal, electromagnetic waves are induced at one specified beam port by magnetic coupling, the induced waves are distributed to the antenna ports through respective channels having different lengths. Therefore, the groups of waves at the antenna ports have respective phases different from one another. In response to these waves at the antenna ports, an array antenna having antenna elements connected with the respective antenna ports forms a transmitting beam. This beam is composed of groups of electromagnetic waves having phase differences. Then, the array antenna radiates this beam in a radiation direction corresponding to these phase differences.
Therefore, each beam port corresponds to one radiation direction of the beam, and the antenna device can radiate a beam in any of radiation directions corresponding to the beam ports.
The antenna device further has a receiving antenna array and a Rotman lens in a beam receiving block. This lens has antenna ports and beam ports. When an incoming beam comes to this antenna array from an incoming direction, antenna elements of the array receive respective groups of electromagnetic waves composing this beam on an antenna surface. The groups of electromagnetic waves at the antenna elements have phase differences corresponding to the incoming direction. Then, in response to this beam, groups of electromagnetic waves having these phase differences are induced at the antenna ports of the Rotman lens by magnetic coupling and are transmitted through respective channels having different lengths to have the same phase at one beam port corresponding to the phase differences. That is, the group of induced waves are combined with one another at the beam port, and a reception signal is produced from the combined waves at the beam port. Because the phase differences of the groups of waves composing the beam corresponds to the incoming direction, each beam port of the lens corresponds to one incoming direction of the beam. Therefore, the antenna device can receive a beam coming from any of incoming directions corresponding to the beam ports.
Accordingly, the antenna device can detect the bearing angle to an object from the reception signal which is produced from a beam coming from any of directions corresponding to the beam ports.
The antenna device performs the beam scanning to radiate a beam, formed by using the Rotman lens, at a scanned angle denoting the radiation direction while changing the scanned angle with respect to time. The number of scanned angles is equal to the number of beam ports. Therefore, the scanned angles are discretely set, and the antenna device performs bearing detection while discretely changing the scanned angle of the scanning beam. In this case, the bearing resolution undesirably becomes low. To heighten this resolution, it is required to increase the number of beam ports. However, the size of the Rotman lens is increased with the number of beam ports, so that it is difficult to manufacture a small-sized antenna device while heightening the bearing resolution in the bearing detection.
To solve this problem, Published Japanese Patent First Publication No. 2003-152422 has proposed an antenna array device. A beam radiated in a particular direction generally has a radiation pattern of electric power with respect to the radiation direction. That is, radiation energy of the beam is maximized in that particular direction, and the beam has also radiation energy in directions surrounding the particular direction. In this device, two beam ports adjacent to each other are changeably selected from many beam ports of a Rotman lens, electromagnetic waves distributed from one selected beam port to antenna ports of the Rotman lens are added with electromagnetic waves distributed from the other selected beam port to the antenna ports, and a transmitting beam induced from the added waves is radiated. Therefore, the beam has a radiation pattern having the maximum radiation energy in the first direction corresponding to one selected beam port and another radiation pattern having the maximum radiation energy in the second direction corresponding to the other selected beam port. The sum of the radiation patterns has a composite pattern having the maximum radiation energy in a third direction placed between the first and second directions. Therefore, the transmitting beam is substantially radiated in the third direction.
Therefore, this conventional device can set scanned angles of which the number is larger than the number of beam ports. Further, this conventional device can also detect each of received beams coming from different directions of which the number is larger than the number of beam ports. Accordingly, the bearing resolution can be heightened in the bearing detection without increasing the number of beam ports.
However, this conventional device requires many selecting switches and a selection controller to appropriately select two beam ports from a large number of beam ports. Because the selection of the beam ports is performed in a cycle corresponding to a frequency in a wide frequency band from several hundreds MHz to tens GHz, it is difficult to manufacture many switches operable in this operating cycle with uniform characteristics. Therefore, it is difficult to manufacture the conventional device operable with high precision.